The present invention is directed toward data communications and, more particularly, a method and apparatus having improved immunity from infrared noise from fluorescent lights and other sources.
Infrared wireless data communication is a useful method for short range (in the approximate range of 0-10 meters) wireless transfer of data between electronic equipment; such as, cellular phones, computers, computer peripherals (printers, modems, keyboards, cursor control devices, etc.), electronic keys, electronic ID devices, and network equipment. Infrared wireless communication devices typically have the advantages of smaller size, lower cost, fewer regulatory requirements, and a well defined transmission coverage area as compared to radio frequency wireless technology (i.e. the zone of transmission is bounded by physical walls and therefore more useful in an office environment). In addition, infrared wireless communication has further advantages with regard to reliability, electro magnetic compatibility, multiplexing capability, easier mechanical design, and convenience to the user as compared to cable based communication technology. As a result, infrared data communication devices are useful for replacing 0-10 meter long data transfer cables between electronic devices, provided that their size and costs can be reduced to that of comparable cable technology.
Infrared data communications devices typically consist of transmitter and receiver components. The infrared data transmitter section consists of one or more infrared light emitting diodes (LEDs), an infrared lens, and an LED current driver. A conventional infrared data receiver typically consists of an infrared photodiode and a high gain receiver amplifier with various signal processing functions, such as automatic gain control (AGC), background current cancelling, filtering, and demodulation. For one-directional data transfer, only a transmitter at the originating end and a receiver at the answering end is required. For bi-directional communication, a receiver and transmitter at each end is required. A combined transmitter and receiver is called a transceiver.
In typical high volume applications, it is now standard practice to fabricate the receiver circuitry and transmitter driver in a single integrated circuit (IC) to produce a transceiver IC. In turn, a transceiver IC, infrared photodiode and LED along with lenses for the photodiode and LED are assembled together in a plastic molded package designed to be small in size and allow placement in the incorporating electronic device so as to have a wide angle of view (typically through an infrared window on its case). The transceiver IC is designed to digitally interface to some type of serial data communications device such as an Infrared Communication Controller (ICC), UART, USART, or a microprocessor performing the same function.
A representative example of a conventional infrared data transmitter and receiver pair is shown in FIG. 1. Infrared transmitter 10 includes LED 16 which generates a modulated infrared pulse in response to transistor 14 being driven by the input data signal DIN. The modulated infrared signal is optically coupled to an infrared detector, such as photodiode 24 normally operated in current mode (versus voltage mode) producing an output current which is a linear analog of the optical infrared signal falling on it. The infrared pulses generated by LED 16 strike photodiode 24 causing it to conduct current responsive to the transmitted input data signal DIN thereby generating a received data signal at DIR.
Data can be modulated on the infrared transmitted signal by any of a number of well known methods. Two of the most popular methods are defined by the Infrared Data Association (IrDA) and Sharp corporation (Sharp ASK). IrDA Physical Layer Link Specification 1.1e specifics two main physical layer infrared modulation methods. One method is a low-speed (2 Kbp/s to 1.15 Mbp/s) on-off infrared carrier asynchronous modulation where the presence of a pulse indicates a 0 bit and the absence of a pulse indicates a 1 bit. The second method is a high speed (4 Mb/s) synchronous Four Pulse Position Modulation (4 PPM) method in which the time position of a 125 ns infrared pulse in a 500 ns frame encodes two bits of information. The Sharp ASK method is similar to the low speed IrDA method but also modulates the infrared carrier with a 500 Khz signal to facilitate differentiating between a valid signal and ambient infrared signals.
In receiver 20, the received signal at DIN is transformed into a voltage signal and amplified by amplifier 26. The signal output from amplifier 26 then feeds into comparator 42 which demodulates the received signal by comparing it to a detection threshold voltage VDET in order to produce a digital output data signal at DOUT.
The received signal waveform will have edges with slope and will often include a superimposed noise signal. As a result, VDET is ideally placed at the center of the received signal waveform so that the output data signal has a consistent waveform width despite the slope of the received signal edges. Also, placing VDET at the center of the received signal improves the noise immunity of receiver 20 because the voltage difference between VDET and both the high and low levels of the received signal is maximized such that noise peaks are less likely to result in spurious transitions in DOUT.
The received signal, however, can vary in amplitude by several orders of magnitude due primarily to variations in the distance between transmitter 10 and receiver 20. The strength of the received signal decreases proportional to the square of the distance. Depending on the range and intensity of the infrared transmitter, the photodiode outputs signal current in the range of 5 na to 5 ma. plus DC and AC currents arising from ambient infrared sources of sunlight, incandescent and florescent lighting. As a consequence, the center of the received signal waveform will vary, whereas VDET must generally be maintained at a constant level. To address this problem, receivers typically include an automatic gain control mechanism to adjust the gain responsive to the received signal amplitude. The received signal is fed to AGC peak detector 36 which amplifies the signal and drives current through diode 32 into capacitor 28 when the signal exceeds the AGC threshold voltage VAGC in order to generate a gain control signal. The gain control signal increases in response to increasing signal strength and correspondingly reduces the gain of amplifier 26 so that the amplitude of the received signal at the output of amplifier 26 remains relatively constant despite variations in received signal strength.
At a minimum, infrared receiver 20 amplifies the photodetector signal current and then level detects or demodulates the signal when it rises above the detect threshold VDET thereby producing a digital output pulse at DOUT. For improved performance, the receiver may also perform the added functions of blocking or correcting DC and low frequency AC ambient (1-300 ua) signals and Automatic Gain Control (AGC) which improves both noise immunity and minimizes output pulse width variation with signal strength.
As noted above, infrared data receivers are vulnerable to infrared ambient noise in their environments. This noise produces spurious outputs and degrades performance by causing bit errors. The predominate sources of noise for infrared receivers in most common environments are (1) photocurrent shot noise from background ambient infrared light; (2) other infrared data transmitters; and (3) fluorescent lights.
Of these three sources, infrared noise from fluorescent lights is typically the most disruptive and most difficult and expensive to mediate. For wideband IrDA devices, receiver optical sensitivity is limited to a value that is as much as 10 times less than is practically possible so as to limit interference from fluorescent lights. Consequently, a low-cost remedy to fluorescent light noise would desirably improve the reliability of infrared communication and allow significantly increased range.
Although the dominant source of infrared ambient light is from sunlight and incandescent lights, this infrared signal is only moderately disruptive to infrared communication since virtually all of the signals produced by these sources are below 200 Hz. As long as the receiver input circuits can handle the DC and low frequency currents produced by these sources, the main source of noise is due to photocurrent shot noise, which is proportional to the square root of the photocurrent.
In an IrDA device, if the total signal to noise falls below about 12 db, then the error rate will become excessive. Consequently, the receiver detect level needs to be set at least 12 db above the maximum shot noise likely to be encountered. If the detect threshold is set too low, then the receiver will produce spurious outputs in high ambient light environments while if the detect threshold is set too high the receiver gain and consequently its range will be significantly reduced to less than is possible in typical light ambients.
A good method for mitigating shot noise so as to prevent spurious receiver outputs is to use a form of adaptive gain control (AGC), as is illustrated in FIG. 1. Since shot noise can be calculated from the photodiode DC current and receiver bandwidth, an effective AGC technique is to measure the DC photocurrent, square root its value, and use the result to set the detect threshold so that detection always occurs at least 12 db above the noise floor. One problem with this technique is that in high light ambients and with typical IrDA photodiodes of 4 square millimeters, the receiver gain may be reduced by several fold, requiring the user to reduce range. However, operation in such high light ambients tends to be infrequent, and the consequent reduction in range is usually acceptable.
In addition to the photodiode shot noise, incandescent lights produce a small noise component modulated at harmonics of the power line frequency of 50-60 Hz. Because the incandescent element has a slow response, there is very little infrared noise radiated above several hundred hertz. For most infrared data receivers, low frequency power line harmonics can be removed by high pass filtering of the amplified receive signal from the photodiode before applying the signal to the signal detector or demodulator. Filtering the low frequency harmonics does not effect reception of the desired infrared signal since most infrared communication signals are at frequencies above 10 Khz. Often, the circuit used to remove the DC ambient component from the photodiode can effectively filter out these low frequency noise components.
Another noise source arises from infrared data transmitters commonly used for a wide variety of wireless communication applications. It is not uncommon to have several different varieties of infrared data transmitters and receivers within receiving range of each other. For example, most consumer electronic remote controls for TVs, VCRs, Stereos, Cable and Satellite TV controls use infrared transmitters. Less commonly, some wireless audio systems use infrared transmitters that produce continuous trains of infrared pulses. Many computer keyboards, cursor pointing devices (mouse, trackball, etc.), and other computer input devices use infrared transmitters to allow wireless operation.
Although these devices use transmit LEDs which radiate on a different infrared wavelength than IrDA transmit LEDs, the difference in wavelength is not sufficient to allow any significant filtering with commonly used low-cost infrared filters. Consequently, these devices have a significant potential to interfere with each other. However, because most infrared control devices operate at low duty cycle and/or with a directed beam, they generally interfere minimally with other infrared devices.
Many infrared communication devices, including IrDA devices, have data transfer protocols that retransmit packets that are lost due to such intermittent interference. However, this technique is not effective against devices that transmit continuously and produce continuous interference, such as infrared based wireless audio systems.
Fluorescent lights produce visible light arising from the narrow band fluorescence of the phosphors inside the fluorescent light tube. The average continuous infrared output from a fluorescent light is significantly less than that of sunlight or an incandescent light for a given visible light output. However, what infrared light is present tends to be significantly noisier in band than infrared light from incandescent lights or sunlight.
Fluorescent light noise arises from the way that the light is generated. In a fluorescent light, phosphors are excited to emit visible light by ultraviolet light radiated from excited mercury vapor ions recombining with electrons. Passing an electric current through the mercury vapor ionizes the mercury atoms. Although most of the light emitted by the mercury vapor is ultraviolet, there is a small infrared component that is also radiated. This infrared light arises from a lesser transition line (1013 microns) radiated by the mercury vapor ion and some of this infrared light leaks pass the fluorescent phosphors.
For fluorescent lights with standard ballasts, AC power line noise may modulate the infrared emitted from the mercury vapor. In addition, the ionized mercury vapor has a non-linear conductivity response, which can readily produce an infrared signal modulated by high frequency harmonics of the power line frequency. For fluorescent lights with high frequency electronic ballasts, the mercury vapor is modulated with harmonics of the electronic ballast switching frequency, which is commonly in the 20 Khz-100 Khz range and consequently can produce significant infrared modulations in these frequency ranges.
Although infrared noise arising from fluorescent lights is limited to frequencies below 200 Khz, this is still within a significant portion of the frequency band of many infrared communications devices, including IrDA devices, and is of sufficient amplitude in many environments to cause disruption of infrared communication. Consequently, it is desirable to mitigate this noise. There are a number of known noise mitigation methods, which have varying effectiveness and cost.
One method for mitigating this type of noise is to use a narrow band optical filter, such as a dielectric filter, to block out the interfering infrared mercury vapor line. Dielectric filters are constructed of layers of light transparent materials having different dielectric constants. The thickness of the layers are usually exact fractional multiples of the wavelength of light being filtered, either to pass or block.
In addition to blocking noise from fluorescent lights, this type of filter can also be very effective against other infrared noise sources. A narrow band optical filter will reduce photodiode shot noise because it reduces the total amount of broadband infrared falling on the photodiode. Because it reduces the total amount of infrared energy it also reduces the total amount of noisy infrared signal passed to the receiver from the photodiode. It can also reduce interference from other infrared devices operating on different wavelengths.
However, despite the highly effective performance of dielectric optical filters, they tend to be very expensive. The cost of a dielectric filter is typically many times the cost of the packaged infrared receiver and photodiode, which raises the total cost of an infrared receiver incorporating a dielectric filter to a level that is typically not competitive against other wired or wireless communication methods. To avoid the cost of dielectric filters, wideband absorption type filters are commonly used instead of narrow-band filters. Although wideband filters pass most infrared frequencies, they will block visible light and prevent noise arising from this part of the spectrum, since photodiodes typically have a very wide spectral response.
Another optical method commonly used to reduce interference noise in infrared receivers is to use lensing and shading. In this approach, the infrared receiver typically has a lens assembly and shading mechanism, which may be part of the case housing the device. The lensing and shading limits the view of the photodiode to the horizontal plane, since infrared transmitters are normally in this plane and interfering light sources are usually above this plane. This method is fairly low-cost, but has only a limited effectiveness, reducing infrared light noise pickup by less than 10 db.
Yet another method used to mitigate noise is to use signal bandwidth filtering in the receiver circuit. Like optical wavelength filtering, the effectiveness of this method depends upon making the filter as narrow as possible. For typical infrared receivers used for remote control, the bandwidth of the signal may be as little as several kilohertz, since these devices demodulate a subcarrier that has a data rate of less than one kilobit per second. However, for IrDA type data receivers the bandwidth is much wider ranging from tens of kilohertz to over 10 Mhz. Consequently, bandwidth filtering for wideband IrDA devices is much less effective. In addition, if the filter is too narrow, then data distortion will result, causing bit errors.
Still another well-known method for mitigating noise is to retransmit packets that are not received correctly. This method is most effective against burst noise and requires noise free intervals between bursts of noise in order to permit the sending of complete packets. Some communication protocols shorten packet lengths dynamically in noisy environments in order to increase the probability of a packet getting through the communications channel. Although retransmission of dynamically sized packets improves immunity to burst noise, such protocols are more complex and more costly to implement in processor resources than is generally desirable for short-range communication. In addition, retransmission of packets is not an effective remedy for continuous noise sources.
Another receiver circuit based method for mitigating noise is the use of some form of adaptive gain control. Adaptive gain control reduces the gain of the receiver in response to input signals according to specific algorithms, so as to produce minimum signal disruption. A common form of adaptive gain control, generally known as Automatic Gain Control (AGC), is used in some infrared receiver systems to both normalize signal levels so as to provide correct pulse width at the detector and to improve noise immunity. As discussed above, receiver 20 of FIG. 1 includes AGC.
Automatic Gain Control works by lowering the receive gain for signals that are typically at least twice the detector threshold. In receiver 20, the voltage level of VAGC determines the signal level at which AGC amplifier 36 begins to reduce the gain, i.e. increase the attenuation, of input amplifier 26. By normalizing the signal at the detector, AGC produces a more uniform pulse width despite large variations in input signal level, and despite long pulse decay typical of a photodiode signal source.
Although it is not immediately apparent, automatically reducing the gain to normalize the signal level at the detector will significantly improve noise immunity. Without gain reduction, any noise above the detect threshold will produce spurious detector outputs between receive pulses even if the receive pulses are much larger than the noise signal. These extra spurious output pulses cause errors in the data stream.
The benefit of automatic gain control occurs when the received signal pulses drive the receiver gain downward so that the noise falls below the detect level. Automatic gain control typically has a fast attenuation attack speed and a slow attenuation recovery process. The AGC attack speed is designed to quickly bring the attenuation of input amplifier 26 up to its final value within a few pulses. The AGC attenuation decay rate is much longer and is typically set to a value intended to sustain significant attenuation between gaps in the data transmission, thus providing significant noise immunity between valid data pulses, preventing spurious outputs.
In receiver 20, the AGC circuit of the receiver is designed such that AGC amplifier 36 rapidly charges up capacitor 28 responsive to high signal levels in order to reduce the gain of input amplifier 26. The charge on capacitor 28 will then slowly decay by discharging to ground through capacitor 30.
For infrared data systems using AGC, in situations where the signal to noise degrades such that communication fails, the intuitive response of the user is to place the receiver and transmitter closer together so as to xe2x80x9cbring them into communication rangexe2x80x9d. This does not decrease the noise, but rather increases the signal level at the receiver, and hence increases the receiver attenuation via the AGC response, which consequently decreases the noise level at the detector. Since infrared systems typically rely on user placement of infrared transmitters and receivers, they normally provide some feedback about whether data transfer is successfully occurring. If transfer is not occurring, then the user moves the receiver and transmitter into range so as to increase the receive signal.
Although automatic gain control is beneficial to suppress noise below the signal level, this is only true so long as a signal is present. Once signal transmission and reception ceases, AGC recovery takes place wherein the receiver attenuation decays to its minimum, as the charge on capacitor 28 drains through resistor 30, at which point ambient noise may cause a spurious output.
This spurious output, if too frequent, will cause disruption of the IrDA protocol. This is because the IrDA protocol is designed to benignly co-exist with other infrared communication systems. The IrDA protocol assumes that any output that it does not recognize is due to some other infrared communication. As a result the IrDA device suspends any attempt at transmitting until it hears no signals for hundreds of milliseconds, so as to prevent interfering with any other infrared communication. However, since the IrDA protocol cannot differentiate between noise and unrecognizable communication, any spurious output recurring with less than a several hundred millisecond interval will prevent communication between IrDA devices, even when they are close enough so that the signal strength suppresses noise with AGC action.
Despite this limitation, AGC still improves noise immunity since it minimizes data disruption in the presence of noise once an IrDA device starts transmitting. Also, the ambient noise at the transmitting IrDA device may be lower than at the receiving device. Consequently, despite the presence of high spurious outputs from one IrDA device, these may not block the initiation of communication by another IrDA device not likewise experiencing high spurious outputs.
Therefore, it is desirable to improve the reliability and maximum range of IrDA communication by reducing spurious outputs arising from ambient infrared noise while not otherwise compromising receiver performance. Accordingly, there remains a need for a method and apparatus for addressing noise in an infrared receiver to improve performance, but without the drawbacks of the conventional solutions.
In accordance with preferred embodiments of the present invention, some of the problems associated with conventional transmission of data on infrared communications systems are overcome. One aspect of the invention includes a method for suppressing ambient noise on an infrared communications link.